1. Technical Field of the Invention
The present invention relates to the isolation of a palladium-based catalyst from a mixture in which it is dissolved.
2. Description of the Prior Art
More specifically, the invention relates to the isolation of a palladium-based catalyst from a mixture originating from the reaction of hydroxycarbonylation of butadiene to pentenoic acids.
One of the objectives of the process of the invention lies in the isolation of at least a portion of the catalyst containing palladium which is dissolved in the form of organometallic complex in the mixture to be treated, in order to enable this catalyst containing palladium to be recycled into a new butadiene hydroxycarbonylation reaction.
A second objective of the process is to make it possible to isolate at least a portion of the pentenoic acids present in the said mixture.
The hydroxycarbonylation of butadiene and/or of its derivatives, such as especially the allylbutenols like 3-buten-2-ol, 2-buten-1-ol and their mixtures, the compounds of addition of hydrogen chloride to butadiene (chlorobutenes), the main one of which is crotyl chloride, can be performed using water and carbon monoxide at a pressure that is higher than atmospheric pressure and in the presence of a catalyst containing palladium, soluble in the reaction mixture.
Reference may be made, for example, to Patent EP A 0 648 731 for a more detailed description of such a technique, although the present invention is not limited to the treatment of reaction mixtures originating from the process according to this patent.
The reaction mixtures used in the process of the invention contain more or less considerable quantities of the compounds involved in the hydroxycarbonylation reaction and of the compounds formed during this reaction.
Apart from the palladium catalyst, which may be present in various chemical forms, the reaction mixture contains the pentenoic acids formed, especially 3-pentenoic acid, water, hydrochloric acid, in most cases also reaction by-products like butenes or valeric acid, dicarboxylic acids like adipic acid, 2-methylglutaric acid and 2-ethylsuccinic acid, optionally butadiene which has not been converted and any optional solvent used in the reaction.
The invention consists therefore of a process for isolation of at least a portion of the palladium dissolved in a solution also containing at least 3-pentenoic acid, characterized in that the said solution is acidified and stirred with an aqueous solution of hydrochloric acid, so as to obtain two liquid phases, including an aqueous phase containing at least a portion of the palladium.
When the solution to be treated originates directly from a butadiene hydroxycarbonylation operation, it is obviously necessary to eliminate the carbon monoxide pressure before the acidification.
The aqueous solution of hydrochloric acid employed generally contains from 5% to 40% by weight of hydrochloric acid by weight of solution.
In general the solution of hydrochloric acid is added in a proportion of 0.1 to 2 times the volume of the solution to be treated.
The formation of two liquid phases during the acidification may originate simply from the addition of the aqueous solution of hydrochloric acid according to the composition of the solution to be treated.
This is particularly, but not exclusively, the case when the solution to be treated contains an essentially water-immiscible solvent such as an aromatic, aliphatic or cycloaliphatic hydrocarbon or a chlorinated aromatic, chlorinated aliphatic or chlorinated cycloaliphatic hydrocarbon.
Separation into two liquid phases may also be obtained by the addition of a water-immiscible organic solvent. This addition may be performed after the acidification, at the time of the acidification or, if appropriate, before the acidification.
The presence of a water-immiscible organic solvent makes it possible to extract at least a portion of the pentenoic acids present in the solution to be treated.
The temperature at which the solution to be treated is acidified is not really critical for making use of the process. It is thus possible to operate between 0.degree. C. and 230.degree. C. (temperature at which the previous hydroxycarbonylation reaction may be conducted). In practice, however, the operation will take place between 20.degree. C. and 200.degree. C. and preferably between 40.degree. C. and 110.degree. C.
The acidification with hydrochloric acid allows the palladium present in the form of organometallic complex in the solution to be treated to be converted into palladium dihydrotetrachloride. The temperature at which the operation is performed affects the rate of this conversion of the palladium compounds, a higher temperature accelerating this conversion, but running the risk of precipitating a proportion of the palladium.
The organic solvent employed for performing the extraction is chosen advantageously from aromatic, aliphatic or cycloaliphatic hydrocarbons and chlorinated aromatic, chlorinated aliphatic or chlorinated cycloaliphatic hydrocarbons which are liquid in the operating conditions and are essentially immiscible with water.
For convenience, the boiling point of the organic solvent will be lower than that of 3-pentenoic acid.
Nonlimiting examples of these solvents which may be mentioned are benzene, toluene, xylenes, chlorobenzenes, cyclohexane, butadiene, butenes, alkanes such as hexanes, heptanes, octanes, nonanes, decanes, undecanes, dodecanes, and the various mixtures of several of these solvents.
The mixture obtained after acidification of the solution to be treated and, if appropriate, after addition of the organic solvent and stirring, separates at rest into an organic phase and an aqueous phase.
As in the case of the acidification, the extraction with the organic solvent may be performed at a temperature situated between 0.degree. C. and 230.degree. C., more often between 20.degree. C. and 200.degree. C. and preferably between 40.degree. C. and 110.degree. C.
The organic phase contains more than half of the initial quantity of pentenoic acids, most of the butadiene and of the butenes and a portion of the dicarboxylic acids which may be present in the solution to be treated.
The aqueous phase contains more than half of the quantity of palladium as well as a portion of the dicarboxylic acids which may be present in the solution to be treated.
The operation of extraction with the aid of an organic solvent may be repeated a number of times if desired.
Depending on the organic solvent employed, the quantity of pentenoic acids in the organic phase may exceed 60% and even 75% of the quantity initially present in the solution to be treated. The pentenoic acids, and more particularly 3-pentenoic acid, may next be isolated from the organic phase by the usual means of chemistry.
The aqueous phase generally contains more than 60% and preferably more than 80% of the palladium initially present in the solution to be treated. With the process of the invention it is even possible to recover in the aqueous phase up to virtually all of the palladium.
The aqueous phase containing the palladium may advantageously be recycled into a new butadiene hydroxycarbonylation reaction. It is generally desirable to distill off beforehand a proportion of the hydrochloric acid which it contains so as to adjust the quantity of hydrochloric acid to the quantity which it is convenient to have for the hydroxycarbonylation. A solution of hydrochloric acid corresponding to the water/hydrogen chloride azeotrope is obtained by such a distillation. In the context of a continuous industrial process it is particularly advantageous to employ the hydrochloric acid solution thus obtained for acidifying the initial solution to be treated, after addition of the top-up which may be necessary.
An alternative form of the process of the invention consists in distilling off at least a portion of the pentenoic acids from the solution to be treated before carrying out the acidification.
Such a distillation will be performed at a temperature lower than or equal to 110.degree. C. This temperature restriction is important because it has been observed that, if the operation is carried out at a higher temperature, a portion of the palladium precipitates. Even when partial, such a precipitation is unacceptable in an industrial process. In effect, it gives rise to losses of a very costly metal and in addition it seriously complicates the treatment of the reaction mixtures.
Unexpectedly, if the operation is carried out at a temperature which is lower than or equal to 110.degree. C., and still more preferably at a temperature which is lower than or equal to 105.degree. C., no precipitation of palladium is observed.
In order to be able to conform to this upper temperature limit, it sometimes suffices to operate at atmospheric pressure. More often, it is necessary to distill the pentenoic acids at a pressure that is lower than atmospheric pressure, generally of the order of 2 kPa to 7 kPa.
The lighter compounds which may be present in the solution to be treated also distill off with the pentenoic acids, like, to give examples, butadiene, butenes, water, possibly a portion of the dicarboxylic acids and the solvent which may be present.
The residue obtained after this distillation contains the palladium and the heavier compounds, like another portion of the dicarboxylic acids. This residue is next treated as described above, with the aid of an aqueous solution of hydrochloric acid generally containing from 5% to 40% by weight of hydrochloric acid by weight of solution. It is not essential to perform the extraction with the aid of an organic solvent as in the first alternative form which has been described.
As already stated, the temperature at which the solution to be treated is acidified may lie between 0.degree. C. and 230.degree. C., in practice between 20.degree. C. and 200.degree. C. and preferably between 40.degree. C. and 110.degree. C.
The acidified aqueous solution containing the palladium may be recycled into a new butadiene hydroxycarbonylation reaction as previously, if appropriate after distillation of the excess hydrochloric acid.
In an equivalent quantity, recycled palladium exhibits a catalytic activity similar to that observed with a new catalyst.
Besides the recycling, in a homogeneous form, of the catalyst containing palladium and the isolation of at least a portion of the pentenoic acids formed, the process of the invention permits the removal of a portion of the by-products of the butadiene hydroxycarbonylation reaction, especially of the dicarboxylic acids, the accumulation of which can prove to be detrimental for the said butadiene hydroxycarbonylation reaction.
The examples which follow illustrate the invention.